1. Field
The disclosed concept pertains generally to energy generating systems and, more particularly, to photovoltaic (PV) systems including PV string maximum power point tracking and PV inverter maximum power point tracking (MPPT). The disclosed concept further pertains to methods of providing MPPT.
2. Background Information
Renewable energy systems, such as photovoltaic (PV) systems, have been widely developed. A PV system includes a plurality of PV modules (e.g., without limitation, typically 200 to 300 W, 8 to 10 A short circuit current, and 30 to 50 VDC open-circuit voltage). Typically, 10 to 20 PV modules are electrically connected in series to form a PV string. In turn, plural PV strings are electrically connected in parallel to form a PV array.
A PV module has an optimal output direct current (DC) voltage under which the PV module can generate maximum power. If the PV modules in a PV string are exactly identical, then the optimal voltage of the PV string for maximizing string output power is the product of the common optimal voltage of the PV modules and the count of the PV modules in the PV string. If the PV strings in a PV array are exactly identical, then the optimal voltage of the PV array for maximizing array output power is the same as the common optimal voltage of the PV strings.
In operating a PV array, maximum power point tracking (MPPT) is generally used to automatically determine a voltage or current at which the array should operate to generate a maximum power output for a particular temperature and solar irradiance. Some PV systems provide a DC-DC converter for each PV module in the PV array. Each of these DC-DC converters performs MPPT to find a maximum power point for its corresponding PV module.
U.S. Pat. No. 8,139,382 discloses an energy generating system including local MPPT for each PV module and centralized MPPT for the entire PV array. The energy generating system comprises a plurality of energy generating devices (EGDs) comprising PV panels, each coupled to a corresponding local converter, that together form an energy generating array. A DC-AC converter coupled to the local converters is capable of receiving current and voltage from the local converters. Each local converter is capable of providing local MPPT for its corresponding PV panel. In this way, each panel may operate at its own maximum power point (MPP) under both ideal and mismatched or shaded conditions.
The energy generating system provides a system control loop for the entire system that is controlled by a central MPPT control block and a local control loop for each of the PV panels that is controlled by the corresponding local converter. The operating frequencies of these loops are spaced apart at least a predefined distance from each other in order to prevent system oscillations and to prevent the panels from operating away from their MPPs. For one embodiment, the system control loop is a closed-loop system that comprises the array, the central MPPT control block, and the DC-AC converter. In addition, each local control loop is a closed-loop system that comprises a panel and its corresponding local converter. For some embodiments, each local converter is designed such that the settling time of the local control loop for that converter is faster than a time constant for the system control loop. In a particular embodiment, the settling time of each local control loop is at least five times faster than the time constant of the system control loop.
U.S. Pat. No. 8,139,382 further discloses that the local converter comprises a power stage and a local controller, which further comprises an MPPT module and an optional communication interface. The power stage may comprise a DC-DC converter that is capable of receiving as inputs a panel voltage and current from the PV panel and reshaping the voltage-to-current relationship of the inputs to generate an output voltage and current. The communication interface of the local controller is capable of providing a communication channel between the local converter and a central array controller. However, for embodiments in which the local converter does not communicate with a central array controller, the communication interface may be omitted.
A simple configuration of a PV system includes a central PV inverter electrically connected to an entire PV array. The central PV inverter repetitively changes the output DC voltage of the PV array in order to track maximum power of the PV array (i.e., MPPT). However, due to a mismatch of PV modules and PV strings, the optimal voltages of the PV strings might not be the same as the optimal voltage of the PV array. Also, the optimal operating points of the individual PV modules are not the same due to mismatch.
In additional to the PV central inverter, various power electronic converters have been added into PV strings or PV modules in order to increase the freedom to search maximum power points for PV strings or PV modules. Table 1 shows four known different architectures of additional power electronic converters and their maximum power point searching capability.
TABLE 1ArchitecturePV ArrayPV StringPV ModuleEaton ®Central inverterS-Max ™provides MPPTsearching forthe entire PVarraySatcon ®Central inverterString DC-DCSolstice ®maintainsconverterconstant DCprovidesbus voltageMPPTsearching forthe PV stringNationalCentral inverterModule DC-DCSemiconductor ®provides MPPTconverterSolarMagic ™; orsearching forprovides MPPTTigo Energy ®the entire PVsearching forModule Maximizerarrayindividual PVmodulesSolarEdge ® PowerCentral inverterModule DC-DCOptimizermaintainsconverterconstant DCprovides MPPTbus voltagesearching forindividual PVmodules
FIG. 1 shows the first architecture of Table 1 for a PV system 1 including a central PV DC/AC inverter 2, which provides MPPT searching for entire PV array 4. The DC/AC inverter 2 keeps searching for the maximum power point (MPP) by changing the DC bus voltage, VDC. In this architecture, the PV string voltages are equal to VDC. The PV array 4 has a plurality of parallel PV strings 6, each with a plurality of series electrically connected PV modules 8.
There is room for improvement in PV systems.
There is also room for improvement in methods of providing maximum power point tracking.